Elucidating Anomalous Intensity Ratios in Chlorine L-Edge X-ray Absorption Spectroscopy: Multiplet Effects and Core Rydberg Transitions.

A relativistic core-valence-separated equation-of-motion coupled cluster (CVS-EOM-CC) study of chlorine L-edge X-ray absorption near-edge structure (XANES) spectra using CHCl and CHICl as representative molecules is reported. The nearly identical intensity for the main features in the L- and L-edge XANES spectra is attributed to multiplet effects and the overlap between core-valence and core Rydberg transitions. The multiplet effects originating from the interaction between the core hole and the C-Cl σ* orbitals account for around half of the deviation of the L and L intensity ratio from the 2:1 ratio of the numbers of 2p and 2p electrons. The 2p → 4s core Rydberg transitions are shown to overlap with the 2p → σ* transitions and contribute to the other half of the intensity anomaly. We demonstrate that triple excitations in CVS-EOM-CC calculations play important roles in accurate simulation of the overlap between the 2p → σ* and 2p → 4s transitions.